The present invention relates to service braking systems, especially for the wheels of large, heavy-duty materials handling vehicles and more especially to such systems that are fluid and spring-operated.
Most engine-powered land vehicles have both a service braking system for variable application during the vehicle's operation to control its speed and stop the vehicle when necessary and a separate and independent emergency or parking brake system for use when the service braking system becomes disabled or when engine power is shut off. Furthermore, the service brake system is typically redundant in that its design allows for the failure of any single component without causing failure of the complete system. In passenger vehicles the service braking system typically comprises hydraulic fluid pressure-applied friction brake shoes or discs which retard or stop rotation of wheel brake drums. There are two separate, isolated circuits; the only common component is the pedal. Failure in one circuit will not affect the other. The emergency brake in such vehicles is typically a manually applied cable-operated means for applying the same brake shoes or discs.
The braking systems of large heavy-duty materials handling vehicles typically have fluid-operated wheel service brakes. They may be either dry caliper disc brakes or wet disc brakes, the latter comprising a wet pack of alternating friction discs and plates, the discs rotating with the wheel and the plates remaining stationary with the brake housing, and with the plates and discs being pressed together by a fluid pressure-operated piston to retard or stop wheel rotation. In such vehicles the emergency or parking brake system is usually independent of the service brake system and designed to apply automatically to stop the vehicle upon failure of the service brake system during vehicle movement. Again, the service brake system is "split;" that is, redundant.
One type of emergency braking system commonly used with such materials handling vehicles is a driveline brake in which friction braking elements are spring-applied to the driveline and released by hydraulic brake fluid pressure tapped from the vehicle's hydraulic service brake circuit. Thus, when hydraulic brake fluid pressure becomes unavailable, either through failure of the service brake system or loss of engine power, the driveline brake is automatically spring-applied to stop the vehicle. Such a system, for example, has been used by Wagner Mining Equipment Co., a division of PACCAR, Inc., for many years in its underground mining vehicles.
More recently, some manufacturers of materials handling vehicles have sought to combine the previously-described wheel service brake with an emergency or parking brake in a single wheel brake housing. These combined service and emergency braking assemblies use the same pack of friction elements previously described as the primary friction braking elements. The assembly typically has a first, service brake piston that is fluid-operated to apply a controlled variable pressure to the pack of friction elements to retard or stop rotation of the wheel. A second, emergency brake piston in the assembly is normally disengaged from the first piston by brake fluid pressure. However, upon failure of brake fluid pressure, a mechanical spring urges the second piston against the first piston to cause the first piston to engage the pack of friction elements to stop wheel rotation. In effect, the described system simply transfers the driveline emergency brake previously described to the wheel brake housing and spring-applies the emergency brake piston to the service brake piston rather than to the driveline. However, such a combined service-emergency wheel brake necessarily requires a bulky wheel brake housing and a complex wheel brake assembly because of the dual pistons and fluid pressure chambers required. The combined dual piston service-emergency hydraulic wheel braking system described is disclosed, with variations, for example, in prior U.S. Pat. Nos. 3,927,737 to Prillinger et al, 4,358,000 to Cumming and 3,941,219 to Myers. An air-operated variation of the dual piston combined service-emergency wheel brake is disclosed in Houser U.S. Pat. No. 3,946,837.
From the foregoing it will be apparent that there is still a need for a simplified fail-safe service braking system for the wheels of a vehicle which eliminates the bulk, weight, complexity and redundancy of prior such braking systems and also eliminates the need for a separate emergency or parking braking system. Therefore, primary objectives of the present invention include:
1. a fail-safe wheel service brake that eliminates the need for complicated and redundant service brake systems; PA1 2. a fail-safe wheel service brake that eliminates the need for a separate and redundant emergency or parking brake; PA1 3. a brake as aforesaid that eliminates the need for combining separate service brake and emergency brake elements in a single wheel brake housing; PA1 4. a simplified wheel service brake as aforesaid that can be retrofitted to existing vehicle wheel, housing and frame combinations; PA1 5. a wheel service brake as aforesaid that is spring-applied and brake fluid pressure-released; PA1 6. a wheel service brake system in which the wheel brake is automatically applied to stop the vehicle upon loss of either brake fluid pressure or transmission fluid pressure; PA1 7. a wheel brake system as aforesaid that provides at least as high braking torque as prior braking systems; PA1 8. a wheel brake system as aforesaid that requires no more and possibly less hydraulic operating pressure than prior systems; and PA1 9. a wheel brake system in which each wheel of the vehicle carries its own self-contained and independently operable spring-applied combination service and emergency brake for optimum simplicity, effectiveness and safety, the hydraulics of the system serving only as a means for controllably releasing such brakes so that upon a loss of hydraulic pressure in the system, the brakes are automatically applied to stop the vehicle.